The identification of radiation-induced base modification is being accomplished through high pressure liquid chromatographic (HPLC) analysis of high molecular weight DNA after its enzymatic digestion to 2'-deoxyribonucleosides. Of particular interest are the modified bases 5, 6, dihdyroxy 5, 6, dihydrothymine (thymine glycol, TG) and 5-hydroxymethyluracil (HMU). TG seems to be subject to enzymatic repaire white the repairability of HMU is uncertain. The repairability of TG will be studied in mammalian cells and using DNA containing TG moieties as a substrate for putative repair enzyme activities. The repairability of HMU will be studied by measuring its disappearance from high MW DNA after irradiation and also in cells to which the (3H) 2'-deoxyribonucleoside (HMdU) is administered. This will determine the role of this radiation-induced derivative in the toxic and carcinogenic effects of radiation. The selective introduction of this derivative into DNA offers the opportunity to analyze the long and short term effects of heterogeneous radiation damage to DNA. The possibility that HMU is responsible for cell transformation will be studied using mouse fibroblasts. The use of HMU as a marker of radiation damage will be investigated by developing techniques for labelling the 2'deoxyribonucleoside (HMdU) with 32P. This may also be done for the 2'-deoxyribonucleoside, dTG. If sufficiently sensitive, this will be a means of quantitatively assessing radiation damage to cellular DNA. Immunologic techniques will also be developed by raising antisera against both the TG and HMU moiety using immunogens consisting of dTG-phosphate and HMUriboside coupled to protein carriers.